Folded and roll paper toweling, such as that used in commercial, “away-from-home” dispensers, is a relatively modest product normally sold almost exclusively on the basis of cost since the purchaser is rarely the user. Because improved performance rarely justifies even a minimal increase in cost, techniques for improving the quality of this product have previously centered around those satisfying the most stringent of economic criteria. Recent market trends have seen a shift toward improved product characteristics; however, economics are still closely monitored.
Traditionally, the production of away-from-home toweling occurs by one of three basic technologies: (i) conventional wet press technology with wet creping and embossing; (ii) conventional wet press technology with dry creping and embossing; and most recently (iii) through-air-drying without creping. Each of these technologies has its own advantages and disadvantages.
Conventional wet press technology with wet creping and embossing results in a product having good strength when saturated with aqueous liquids. This technology suffers from the disadvantage that the product lacks sufficient absorbent capacity and softness. As described in U.S. Pat. No. 5,048,589 to Cook et al., herein incorporated by reference in its entirety, towels made from a conventionally wet pressed, wet crepe process “are normally strong even when saturated with liquid, but often lack desirable levels of absorbent capacity, absorbent rate, and softness.”
Conventional wet press technology with dry creping and embossing results in a product having good absorbent capacity and softness; but the product lacks strength when saturated with aqueous liquids. U.S. Pat. No. 5,048,589 to Cook et al. describes products made by this method as “ . . . soft towels [that] possess high levels of absorbent capacity and absorbent rate, however, these soft towels are also very weak and tend to break apart when saturated with liquid.”
Through-air drying without creping is also disclosed, for example, in U.S. Pat. No. 5,048,589. The '589 patent discloses towels with good absorbent capacity and strength when saturated with an aqueous liquid. Uncrepe technology as described in the '589 patent was developed to overcome some of the difficulties in making soft, strong, and absorbent wiper towels.
Although through-air-drying with both creping and embossing can result in a product that is relatively soft and absorbent, this product is generally regarded as a retail in-home towel because of its marginal strength. For example, a particularly successful through air dried towel marketed as a retail in-home product is two-ply Bounty®. Two successful high quality away-from-home folded towels are single-ply KC Surpass® 50000 and Scott Select® 189. The geometric mean wet tensile strength of Bounty® is approximately 895 g/3″, while the geometric mean wet tensile strengths of KC Surpass® 50000 and Scott Select® 189 are generally 1297 g/3″ and 970 g/3″, respectively. Clearly, conventional retail in-home through-air dried towel products are lower in strength. So, for applications where strength is an important consideration, e.g., in the area of away-from-home toweling, traditionally through-air-drying is not coupled with operations that lead to a decrease in strength, for example, dry creping or embossing.
The present invention provides a method of overcoming the disadvantages associated with each of the prior art technologies. The method according to the present invention produces a single-ply towel using through-air-drying, creping, and embossing that does not suffer from the marginal strength of prior art towel products while maintaining both high softness and good absorbency. This is accomplished through the use of an anionic/cationic thermally cross-linking strength additive system at a headbox charge controlled to a specific anionic range; preferably in conjunction with a furnish having as its major component, refined long fibers; and high levels of wet strength/dry strength resins.
Prior art through-air-drying processes do not provide a method for making a strong, soft, and absorbent away-from-home hand drying towel using high levels of refined softwood, adding high levels of wet strength resin, and adding wet/dry strength resins to appropriately control headbox charge to a specified anionic range.
U.S. Pat. No. 3,998,690 to Lyness et al., incorporated herein by reference in its entirety, discloses a chemical flocculation technique for using short fiber to make bulky webs. Flocculation of the furnish tends to produce aggregates that apparently cause a short fiber furnish to act like a long fiber furnish. Lyness et al. discloses the use of wet strength resins or other cationic agents and anionic agents for inclusion in a bifurcated furnish which requires the use of a complex stock system. Although Lyness et al. discloses that a stoichiometric charge density balance of the anionic/cationic pairs can be used, they do not include the furnish as part of the charge balance. Furthermore, measuring and controlling headbox charge to a specific anionic range for improved wet strength is not considered by Lyness et al.
There are numerous schemes for measuring the charge state of a wet end system. Two of the most common methods are described below: zeta potential via micro-electrophoresis and titratable charge.
When a negatively charged particle, such as a wood pulp fiber, is suspended in an aqueous solution, the negative surface attracts a considerable number of positive counterions next to the electrified interface. The counterions next to the electrified interface are strongly attracted into a thin layer referred to in the literature as the Stern layer. When a particle moves in solution, liquid immediately adjacent to the particle surface moves with the same velocity. This unknown boundary layer is referred to as the shear surface and contains the Stern layer. Therefore, in a fiber furnish, solution and counterions are bound to the moving electrified fiber particle in the shear/Stern layer.
Counterions tend to diffuse away from an electrified surface because of thermal motion, but they are also attracted by coulombic forces. These opposing effects cause charge concentration variations which effect the double layer potential in solution. Zeta potential is the double layer electrical potential at the shear surface. Salts added to a solution suppress the electrical potential or double layer potential in solution, and thus, reduce the zeta potential without changing the charge on the particle.
The most common technique for measuring zeta potential is by microelectrophoresis. Microelectrophoresis techniques require a particle dispersion to be placed in a cell and an electric field applied. The velocity of the particles is determined, e.g., microscopically. The mobility is calculated as the particle velocity per unit electric field. The zeta potential is then calculated from the Helmholtz-Smoluchowski equation as the mobility times the viscosity of medium divided by the dielectric constant of medium.
The electrostatic charge associated with papermaking particles and polyelectrolyte additives defines the cationic or anionic demand of a papermaking system. The most popular technique for measuring the state of charge of a wet end system is to titrate a papermaking sample, like a headbox sample, with known concentrations of standard cationic or anionic solutions. Frequently, the end point of the titration is zero streaming current or zero electrophoretic mobility. (The streaming current detector is an instrument used for characterizing colloidal surface charge by measuring the current generated by mobile counterions when charged material adheres to piston and cup walls while the piston moves.) The amount of standard charged material needed to neutralize the papermaking or headbox sample gives the charge state of the system.
Details on both the electrophoretic mobility and titratable charge techniques can be found in Principles of Colloid and Surface Chemistry by P. Hiemenz and in Chapter 4: Application of Electrokinetics in Optimization of Wet End Chemistry in Wet Strength Resin and Their Application (L. Chan, Editor, 1994).
The combined use of cationic and anionic strength adjusting agents to enhance the strength properties of paper webs has been the subject of much discussion. Charles W. Neal, A Review of the Chemistry of Wet Strength Development in 1988 Tappi Seminar Notes describes several commonly utilized wet strength additives, their preparation and chemical structure, their cross-linking reactions, and their effect on wet strength properties. This review includes a discussion of cationic/anionic additive systems such as the PAE/CMC (polyamidepolyamine-epichlorohydrin/carboxy methyl cellulose) system. Neal describes the cationic additive as acting as a retention aid for the anionic additive. Neal discloses wet end chemistry parameters for optimum wet strength properties for the PAE resin system as including operation of the wet end at a pH level that is neutral to slightly alkaline with minimization of free chlorine via the use of an antichlorine agent.
Early development of a PAE/CMC system is described, for example, in U.S. Pat. No. 3,058,873 to Keim et al., assigned to Hercules. Keim et al. discloses a process for the production of improved wet strength paper using PAE type cationic resins and water soluble gums selected from the group consisting of water-soluble cellulose ethers (e.g. CMC) and cationic starches. Keim et al. state the improved wet strength from the PAE/CMC system is due to a synergistic effect involved when PAE and CMC are used in combination. Subsequent work by Hercules is described in, for example, Herbert H. Espy, Poly (Aminoamide)—Epichlorohydrin Resin—Carboxy Methyl Cellulose Combinations for Wet and Dry Strength in Paper, 1983 Papermakers Conference Proceedings. Espy discusses the mechanism by which CMC contributes to retention of PAE beyond the simple demand by the pulp, thus improving not only wet strength but also dry strength of the paper web. For example, when CMC is added to a system containing high levels of PAE, a less cationic coacervate is formed, enabling more PAE to be deposited on the fiber. If excessive levels of CMC are added, anionic coacervates are formed which are not adsorbed onto the pulp fibers. This added retention is referred to by Espy as the synergy of these two strength additives. Espy describes electrophoretic mobility as a basis for determining optimum CMC/PAE ratios. Espy does not address the effect of the charge on the headbox furnish as a means for controlling and optimizing strength additives to a paper web and the resultant web properties.
Three methods for investigating charge in fiber suspensions are described in Practical Experiment with Determination of Ionic Charges in Paper-Machine Circuits by M. Wolf. The article which is incorporated herein by reference was published in Wochenblatt fuer Papierfabrikation, Vol 118, No. 11/12, pp. 520–523, June, 1990. The methods reviewed were polyelectrolyte titration (PE) with o-toluidine blue (TBO) as an indicator, polyelectrolyte titration using the streaming current detector (SCD) signal as the endpoint and electrophoresis. PE with TBO as an indicator measures the anionic and cationic demand of pulp slurries and filtrates via a back titration scheme which is plagued with procedural problems of altering the sample with distilled water and precisely determining the end point value visually. This technique was used in a paper board mill operating with native starch. Table 2 in this article shows that the headbox charge was in an over cationization state—outside the range of interest for operating a wet strength system on a towel and tissue paper machine. Also, Table 3 in this article shows that the addition of cationic starch increases the cationic nature of the mixing chest stock. For this example, no mention of controlling and measuring headbox charge in the range of less than about 0 to −115 meq×10−6/10 ml is made when cationic starch is added. Also, cationic materials like wet strength resins and anionic materials like dry strength agents were not added, and the rate was not set so that headbox charge was adequately constrained.
The second technique for measuring stock charge conditions described in Wolf's article uses polyelectrolyte titration with the SCD to determine end point. This technique is a substantial improvement over the PE/TBO method. The specific anionic consumption (SAC) and specific cationic consumption (SCC) are outputs of the test. Since samples are not diluted with water, the ionogenity of the solution is maintained.
Examples in Table 4 of Wolf's article show the analysis of anionic trash in a groundwood containing coated paper machine using PE/SCD. Cationic fixing agents were used to eliminate anionic trash. The headbox charge was measured and reported to be extremely negative. The values are clearly outside the range of interest for operating a wet strength system on a towel and tissue paper machine.
Table 5 shows PE/SCD results when cationic starches are used. Addition of cationic starch, especially starch B, increases bond strength. Headbox charge was not measured.
In one example in Table 5 and in another example in Table 6 of Wolf's article cationic starch is added in combination with anionic starch. White water PE/SCD values were measured. For the data in Table 5 the white water PE/SCD value increased (i.e. moved from a negative value to a less negative value) with a slight increase in bond strength. The data in Table 6 shows a decrease in white water PE/SAC values (i.e. moves from a positive value to a less positive value) with a corresponding increase in bond strength. Headbox charge was not measured. This article does not disclose the use of cationic wet strength agents/anionic dry strength agents as a means to maximize wet strength properties for a non-compacted hand drying towel. Furthermore, data from Table 5 does not disclose controlling and measuring headbox charge in the range of less than about 0 to −115 meq×10−6/10 ml by controlling anionic/cationic starch levels.
Table 7 in Wolf's article shows data comparing the PE/SCD measurement with the electrophoretic mobility values. Measurements were made at headbox, cleaner stage, and machine chest. Zeta potential and PE/SCD values show that the system is slightly negative. Although PE/SCD charge values in the headbox are in the range of less than about 0 to −115 meq×10−6/10 ml, the charge was not manipulated by using anionic/cationic additives.
In conclusion, Wolf measures PE/SCD at various points in a paper machine system but fails to show that maximum wet strength occurs when headbox charge is controlled in the range of less than about 0 to −115 meq×10−6/10 ml by appropriately adjusting the cationic wet strength resin content and anionic dry strength resin content.
The P. H. Brouwer article entitled The Relationship Between Zeta Potential and Ionic Demand and How It Affects Wet-End Retention (Tappi Journal/January, 1991, p. 170) describes schemes for optimizing wet end starch retention by optimizing first pass retention via the use of retention aids and by keeping zeta potential and cationic/anionic demand close to zero. In one example of a paper machine making coating base paper from mechanical pulp and CaCO3 filler with 0.5% polyaluminum chloride (PAC) added at the mixing chest, 0.8% cationic potato starch added just before the fan pump, and 0.02% retention aid before the headbox, COD levels exceeded acceptable limits. When PAC was increased to 1% and COD decreased from 200 mg/l to 155 mg/l, headbox cationic demand was reduced to 100 meq×10−6/10 ml (i.e. headbox charge was −100 meq×10−6/10 ml). In a second example, 80 g/m2 packaging paper was made from a furnish consisting of 36% bleached long fiber, 38% bleached short fiber, 20% broke, and 6% filler. Rosin and alum were added at 17.5 Kg/ton and 50 Kg/T, respectively. By adding 1.5% anionic potato starch phosphate, headbox anionic demand decreased to 50 meq×10−6/10 ml (i.e. headbox charge was +50 meq×10−6/10 ml). The addition of anionic potato starch phosphate improved dewatering, gloss and dry tensile strength.
An article by McKague entitled Practical Application of the Electrokinetics of Papermaking in Tappi/December, 1974, Vol. 57, No. 12, p. 101, reviews the application of electrokinetics to photographic papermaking systems. Their experimental data-shows that maximum wet and dry strength occur at −0.75 electrophoretic mobility when a small amount of anionic dry strength resin was added to the photographic papermaking system. The other ingredients in the system are cationic starch, cationic wet strength resin, anionic sizing material, and hydrolyzed aluminum salt. The amount of materials, the types of resins, and where they were added were not disclosed in the article.
An article by Patton & Lee entitled Charge Analyses: Powerful Tools in Wet End Optimization in 1993 Papermakers Conference Proceedings, p. 555, reviews charge analysis schemes: zeta potential, colloid titration ratios and charge demand titrations. The article states that zeta potential is an indirect indication of the density of charges on a particle surface; zeta potential and electrophoretic mobility are measurements of the same material characteristic; and zeta potential has the disadvantage of being ionic strength and temperature dependent. Patton et al. describes charge titration as the second major category of wet end charge analysis methods; however, Patton et al. dismisses charge titration as an effective method of predicting furnish response to wet end chemistries. Patton et al., while disclosing that either monitoring system can flag possible changes in machine performance and efficiency, clearly states that measurement of zeta potential is necessary to accurately predict system response to retention aids.
A case study is presented for the wet end of the alkaline fine paper machine using precipitated calcium carbonate filler, dual polymer retention systems, internal size, and wet end starch. Charge demand titrations showed that the wet end was cationic; the machine suffered considerable deposits which resulted in holes and breaks. The cationic donor in the dual polymer system was slowly reduced; sizing increased while headbox charge became slightly anionic −20 to −60 meq×10−6/10 ml. The article by Patton & Lee focused on sizing systems.
An article by W. H. Griggs and B. W. Crouse entitled Wet End Sizing—An Overview in Tappi/June, 1980, Vol. 63, No. 6, p. 49, reviews the types of sizing materials and the interrelationship of sizing to electrokinetics, pH, and formation. They show that maximum wet and dry strength levels occur at −7 mv of zeta potential for a complicated wet end system containing dry strength agents, brighteners, dyes, size, Al+3, and wet strength agents.
An article by E. E. Moore entitled Drainage and Retention Mechanisms of Papermaking Systems Treated with Cationic Polymers in Tappi/January, 1975, Vol. 58, No. 1, p. 99, shows that optimum drainage or retention of a papermaking system in which a drainage and retention aid is used does not necessarily correlate with the point of zero zeta potential of the substrate surface. In a bleached pulp system containing alum, drainage increases when zeta potential is increased by adding cationic polyacylamide. Furthermore, in a bleach pulp system containing 2 lb/T alum, the addition of 1 lb/T cationic polyacrylamide changed the zeta potential from 0 to +30 mv, while improving permeability by more than 50%. This data was generated with pulp samples refined in deionized water. The polymer treated samples (alum/cationic polyacylamide) were washed and used to measure streaming potential.
An article by E. Sandstrom entitled First Pass Fines Retention Critical to Efficiency of Wet Strength Resin in Paper Trade Journal/Jan. 30, 1979, p. 47, shows that optimum wet strength results were obtained at −6 mv headbox zeta potential for an amphoteric retention aid polymer and at −3 mv headbox zeta potential using a low molecular weight quaternary amine. He concludes that first pass retention can be increased for better wet strength resin performance through zeta potential suppression and through the use of high molecular weight polymers. This article also discloses negative effects of excessive use of retention aids (i.e. positive charge in the headbox): excessive yankee adhesion and felt filling.
An article by Dixit et al. incorporated herein by reference entitled Retention Strategies for Alkaline Fine Papermaking with Secondary Fiber: A Case History in Tappi Journal, April, 1991, p. 107, reviews methods for measuring charge: zeta potential, colloidal titration ratio, and cationic demand. A case study was discussed showing schemes for improving first-pass retention in blue basestock. The highly anionic blue dye was causing system charge unbalance and adversely affecting first pass retention. A cationic low molecular weight, high charge density polyamine polymer was added to the machine chest for total retention and first pass ash retention improvements. System charge was reduced from −25 mv to −13 mv of zeta potential.
An article by C. King entitled Charge and Paper Machine Operation in 1992 Papermakers Conference Proceedings, p. 5, discusses four schemes for measuring charge: electrophoresis, streaming potential, streaming current, and colloidal titration with an end point color change. King does not distinguish one method versus another when describing charge in his article. While King does refer to charge, it is clear that King is, in fact, referring to zeta potential, quantities related to zeta potential or quantities related to the sign of the charge.
Edward Strazdin has written a number of articles discussing the measurement of mobility (related to zeta potential) on fiber furnishes. In the article Entitled Factors Affecting Retention of Wet-End Additives in Tappi, Vol. 53, No. 1, January, 1970, p. 80, Strazdin discusses the role of cationic long chain polymers on retention of emulsion-type sizing agents. He also discusses the colloidal and retention characteristics of melamine formaldehyde wet strength resin and how these characteristics are affected by electrokinetic charge. The experiments were laboratory Noble and Wood handsheet studies and mobility measurements were made on diluted thick stock samples after chemical addition. For a synthetic size based on a cellulose reactive stearic anhydride, the addition of a cationic polyamine caused sizing to maximize at zero mobility. Changing mobility with the addition of sulfate ion or ferricyanide ion led to a maximum in wet tensile strength as zero mobility was approached. Using carboxy methyl cellulose to vary mobility, maximum wet strength occurred at positive mobility, apparently due to particle size variation with charge density changes.
In the article entitled Optimization of the Papermaking Process by Electrophoresis in Tappi, July, 1977, Vol. 60, No. 7, p. 113, Strazdin shows that sizing and wet strength of a photographic grade paper were optimized by balancing, essentially to zero, the electrokinetic mobility through the neutralization of the cationic charge with anionic dry-strength resin. Fiber furnish was high-alpha cellulose bleached sulfite; fatty acid anhydride emulsion was used as the sizing agent; cationic polyamine-epichlorohydrin resin was used as the wet strength agent; and an anionic polyacylamide dry-strength agent was used to balance charge. Experiments were performed on handsheets. Mobility measurements were made on stock filtrate.
In the article entitled Microelectrophoresis Theory and Practice in 1992 Papermakers Conference Proceedings, p. 503, Stradzin shows the importance of microelectrophoresis for optimizing wet-end chemistry. A maximum in wet strength occurs at zero electrophoretic mobility where mobility was varied by adding a cationic promoter to a cationic polyacrylamide system contaminated with a constant level of anionic carboxy methyl cellulose. Another experiment shows that retention maximizes at zero zeta potential when zeta potential was varied by changing cationic guar gum levels. Stradzin criticizes non-zeta potential schemes for measuring wet end chemistry properties, e.g. pad techniques, CTR, saying that they produce results with varying degrees of deviation from the correct values.
In Chapter 4 of Wet Strength Resins and Their Applications (1994, Editor: L. Chan) entitled Application of Electrokinetics in Optimization of Wet End Chemistry, Strazdin thoroughly reviews techniques for measuring electrokinetic charge, e.g. zeta potential, streaming current detector, colloidal titration ratio, and cationic demand. He shows that wet tensile strength is a maximum at zero mobility for a cationic polyacrylamide resin containing varying levels of anionic carboxy methyl cellulose. In an article by Strazdin entitled, Chemical Aids Can Offset Strength Loss in Secondary Fiber Furnish Use, in Pulp & Paper, March, 1984, p. 73, analytical techniques for assessing the effectiveness of chemical additives for improving retention are discussed, including dual polymer retention aid systems. Furthermore, his results show that a dry strength resin is most efficient if added to a long fiber fraction versus a short fiber fraction.
U.S. Pat. No. 5,368,694 to Rohlf et al. discloses a method for controlling pitch deposition from aqueous pulp suspension having neutral or cationic charge defined as −100 meq×10−6/10 ml to +800 meq×10−6/10 ml. The method involves contacting the pulp suspension with a water soluble anionic polymer or anionic surfactant to change pulp suspension charge to at least −150 meq×10−6/10 ml without negatively effecting the quality of paper and further contacting the paper machine equipment surfaces with a water soluble cationic polymer or surfactant that has a charge density of at least 0.1 meq/g. U.S. Pat. No. 5,368,694 argues against maintaining pulp suspension charge from less than about 0 to −115 meq×10−6/10 ml and suggests that aqueous pulp suspension should be maintained at a soluble charge of at least −150 meq×10−6/10 ml, preferably increased to greater than −200 meq×10−6/10 ml and most preferably greater than −300 meq×10−6/10 ml.
U.S. Pat. No. 4,752,356 to Taggert et al. discloses a method for controlling cationic material additives in order to neutralize a papermaking slurry containing anionic contaminants using total organic carbon measurements of samples of slurry as an indicator of cationic demand. Taggert et al. discovered that TOC measurements of filtered papermaking slurry samples correlate with cationic demand of the slurry. They advocate measurement of TOC of slurry samples before final chemical addition. To set limits on TOC for optimal papermaking conditions would require a unique relationship between TOC and cationic charge. A unique relationship of TOC versus cationic demand is not demonstrated in the '356 patent.
The role of zeta potential or the closely related quantity, electrophoretic mobility, for wet end optimization has been a factor of much debate in the literature. Brouwers, previously cited, describes the results of pulp filtrate conductivity experiments where conductivity varied by adding Na2SO4. Brouwers states that, “[a]t low conductivity, a zeta potential of close to zero (e.g., −2 mv) would provide optimum papermaking conditions, because hardly any anionic trash is left (low cationic demand). However, at higher conductivities, disturbing amounts of anionic trash are still present at a zeta potential of −2 mv.” Therefore, setting targets based on zeta potential can lead to conditions where cationic demand is either low or high. As determined in conjunction with the present invention, it is better to set targets based on the system charge.
Another example where setting limits on zeta potential for optimum papermaking conditions lead to system difficulties can be found in an article by Strazdin in Pulp & Paper, March, 1984, p. 73, previously cited. Strazdins discloses that the use of electrokinetic charge or mobility as the sole guideline is only applicable to furnishes that contain low levels of electrolytes, i.e. where the conductivity is low. Strazdins asserts that the arguments become different if the furnish contains high levels of dissolved electrolytes, i.e. the conductivity is high. In that case, the range of coulombic forces is greatly reduced and the magnitude of the mobility decreases to a low value regardless of the extent of stoichiometric charge balance and the amount of dissolved anionic contaminants in the aqueous phase. Strazdin thus suggests that it is difficult to set proper limits on zeta potential for optimum papermaking conditions.
The afore described literature is neither conclusive nor consistent in determining optimized zeta potentials. Based upon the prior art's widely varying optimums in zeta potentials, appropriate operating ranges have been difficult to predict.
The present invention overcomes disadvantages associated with the prior art by providing an effective means for producing a soft, absorbent, strong non-compacted away-from-home hand towel by combining refined long fiber with high levels of cationic wet strength resin/anionic dry strength agents where the cationic/anionic resins are varied so that headbox charge is controlled within a specified anionic range